The present invention relates to a high-strength cold-rolled steel sheet having a composite structure composed of a ferrite and a second phase (mainly martensite), excellent elongation and stretch flange formability, and superior workability, and also relates to a method for production thereof.
For example, steel sheets used for an automobile demands high-strength and excellent molding workability, taking into account both passenger's safety and body-weight reduction for saving fuel consumption. Various strengthening methods are adopted in manufacture of high-strength steel sheets. Especially as high-strength steel sheets strengthened using hard martensitic structure, much attention has been focused on a composite structure steel sheet having a ferrite-martensite two-phase structure.
The present inventors have investigated steel sheets with a composite structure having high-strength and superior workability. The present inventors has already proposed high-strength cold-rolled steel sheets having superior workability described in Japanese Patent Laid-Open (JP-A) Nos. 63-241115, 63-293121, 9-67645, 10-237547. All of these Referential Patents secure workability by a soft ferrite phase as a first phase (main phase) by specifying a content of C, Si, and Mn as basic components, and simultaneously, by using steel including proper quantity of Cr, Mo, etc., and by controlling cold rolling conditions and cooling conditions after hot-rolling, conditions of subsequent heat treatment and aging treatment, etc. They also realize coexistence of strength and workability by securing strength by precipitation of a low temperature transformation forming phase of martensite etc. having structure strengthening effect.
In recent years, it has become clear that adjustment of a hardness ratio and a hardness difference between a ferrite phase and a low temperature transformation forming phase in the steel sheet with a composite structure can improve stretch flange formability (λ) regarded as important for forming workability. In more detail, it has also become clear that a smaller hardness ratio and a smaller hardness difference can further improve stretch flange formability.
JP-A No. 11-350038 discloses a technique wherein a combination of suitable steel compositions and manufacturing conditions give suitable composite structure, and enables production of a cold-rolled steel sheet having superior elongation and stretch flange formability with concurrent secure of high-strength. The JP-A No. 11-350038 specifies a content of Nb, Ti or V as important additional trace elements, and it also clarifies that skillful use of refining effect of crystal grains by fine carbide, caused by addition of these elements, produced in steel gives both of excellent ductility and stretch flange formability.
The steel sheet with the composite structure is excellent in compatibility between a high-strength and excellent elongation and stretch flange formability. However, in recent years, there have been increased demands for thin-walled and light-weighted material steel sheets and yet improved processing efficiency. To cope with this, high-strength steel sheet having excellent elongation and stretch flange formability exceeding the conventional technique level would be needed.